Fiber: Where is the Evidence?


I remember coming across a meta-analysis that found there actually were pretty much no health differences between people who had a bunch of fiber and those who didn’t. And I’m wondering about the gas issues being reported.

@Soylent, where do we stand on the culprit of the gas for most people? The addition of enzymes did not seem to solve the problem for everyone, but I am wondering what the facts are. If most people did indeed have gas issues linked to the amount of fiber in Soylent, what is being done to reduce the amount of fiber in it?

With something evidence-driven like Soylent, the amount of fiber in it confuses me. Is there any strong evidence for a substantial benefit from 20+ grams of fiber? By “strong evidence” I mean evidence shown in meta-analyses and/or systematic reviews. By “substantial” I mean statistically significant >2% differences in relevant human outcomes. After trying a quick PubMed search, I could not find anything that looked promising. In fact, I faintly remember coming across a meta-analysis finding that there actually was pretty much no difference between people who consumed a bunch of fiber and those who didn’t.

So where is the evidence for all the fiber? If there’s enough interest, I’ll dig up the study I mentioned earlier. And please feel free to share links to systematic reviews and meta-analyses in this thread!


I don’t have sources but when we’re talking about a liquid food based in carbohydrate, fiber is going to have a significant impact on the glycemic load of the food. I’m not on a convenient platform to search the forums right now but there was a post awhile back where a user provided charts of his blood sugar after Soylent with and without added psyllium husk, and it was predictably noticeable. Lowering the base amount of fiber will impact this further for the worse.

Of course, if that’s really the only issue then other tweaks can be made to compensate for a reduced ratio of carbohydrate. I don’t know enough to say, so I’ll step aside and let the bigger brains around here take over.


Yes. Please find this study you think you remember.


That is a good point… A few weeks ago I recall having Soylent like amount of farts after eating a lot of Rye bread. (Common food in Denmark) which contains a lot of fiber… I am not saying the fiber caused the farts… But they did.


IIRC @Rob left any significant amount of fiber out of his original experiment saying he didn’t expect he would need it with an all liquid diet, then later added it into the mix after having some issue.

Here’s what he was doing originally… Believe it or not the optimal amount of fiber I found is only 1.2g. I know the FDA recommends much more, but that’s probably assuming a more conventional diet.

Here’s another thread on the subject.


It seems that I’ve found the post where he mentions the fiber increase as a result of the oat powder. It doesn’t seem that he was having a health issue with the 1.2 grams of fiber, though there may have been other factors that influenced his decision.

And I think tomorrow I will try to find that study I mentioned earlier. Soylent has an immense amount of potential, and it will become a force to be reckoned with through evidence based design.


Even if all 400g of carbs from the original recipe had been replaced with oat flour (which they weren’t) it should only account for ~26g of fiber, so he also added quite a bit more fiber than what the oat flour alone provided.

Also from that blog post…

I underestimated the importance of fiber in a diet, and went from consuming 1.2g / day to 40g / day.

While not elaborated there, I seem to remember a subsequent post (here I think) where the reason was given as gut biom health.

I do recall seeing something (an article, maybe an abstract) a while back indicating fiber may not be necessary, but from what I remember it was based on a single study, which is pretty meaningless if that is the case. I look forward to seeing what you’ve got, but I doubt I’ll be leaving the fiber behind anytime soon.


I dug a bit through my notes and found that I was thinking of a few studies which were indeed not meta-analyses.
For what it’s worth, I’ll sum up the findings. The studies were published in 2006 and had about forty nine thousand women participate for eight years. One of the things measured was how a diet modification would impact health outcomes. They didn’t seem to go far out of their way to measure the fiber intake of the women, but here’s a quote from the methods section:
“Briefly, the intervention was designed to promote dietary change with the goals of reducing total fat intake to 20% of total energy and increasing consumption of vegetables and fruit to at least 5 servings daily and grains to at least 6 servings daily. The intervention did not include total energy reduction or weight-loss goals.” [Link to quoted article][1]

What these studies found was that the dietary intervention group had no significantly different rates of breast cancer, colon cancer, or heart disease.
Links to more articles from these studies:
Link to a nice chart with links to smaller studies that these findings seem to contradict:’s_Health_Initiative#Study_components_and_primary_findings

I can cite a couple more studies unrelated to the Women’s Health Initiative stuff above, but they are also not the nice meta-analyses or systematic reviews I was hoping to find. And their sample sizes aren’t even above 10k human subjects :wink:


Don’t summarize, post a link to the study itself and let us draw our own conclusions. :smile: It sounds like the study wasn’t looking at fiber specifically and using it as evidence that fiber may not be needed is a stretch.


I don’t believe anyone here meant to imply that fiber is wholly unnecessary. My central argument is that the amount of fiber may be excessive and the drawbacks of such amounts may far outweigh the perceived potential benefits. That being said, I haven’t looked into studies that show a causal, dose-dependent relationship between fiber and flatulence. If anyone has come across such studies, please feel free to enlighten us!


It’s not impossible I just misunderstood what you where saying. It happens way more than I would like. There is actually a range of recommended grams of fiber per 1000 calories. Admittedly Soylent is at the high end of the recommended range. As with anything some people need and can handle more/less than others. Soylent is a one size fits most product and will never satisfy everyone as is.


All of them mention gas as a side effect of too much fiber in short time… including bloating and stomach upset, even diarrhea. But also that the body can adjust to it.

That being said, even if they body adjusts, it doesn’t mean that it is a better amount of fiber after you adjusted… I am beginning to hope they lower the fiber to a more reasonable level.


I don’t see how those links support reducing the fiber in Soylent. They talk about what happens if there’s too much, but they don’t say that the level in Soylent - 27g - is too much.

They say things like,

“But if you’re getting more than 30 grams of fiber, and are suffering from digestive discomfort, try cutting back a little bit.” -

“20 to 35 grams recommended daily, according to the National Institutes of Health” -

“However, adults who consume the recommended amounts of fiber – 25 grams a day for women and 30 to 38 grams a day for men – are unlikely to have problems with nutrient absorption.” -


fiber is honestly the best candidate for the amount of gas people are experiencing… Regardless of how healthy it is. Sulphur is the reason for the deadly smell.


Eskimos had a diet that was basically 0% fiber and didn’t have problems with diarrhea or constipation, so fiber can’t be that important.


No kidding? I thought Eskimos dealt with constipation and gas all the time.

Their most powerful god, Matshishkapeu, also called the “Fart Man,” would curse people (and other gods) with painful constipation.


Huh, I did not know that. (Hehehe “Fart-Man”)